1. Field of the Invention
The present invention relates to a power limiting apparatus for an electric system, a power limiting method for an electric system and an electric system, and more particularly to a technique of limiting electric power of a power storage system provided with a plurality of power storage devices.
2. Description of the Background Art
Conventionally, an electric vehicle equipped with an electric motor as a driving source is known. Such a vehicle is equipped with a power storage device that stores electric power, such as a battery and a capacitor. Electric power stored in the power storage device is supplied to the electric motor. The electric motor is controlled so as to output desired power.
One problem with the electric vehicle lies in that the distance that the vehicle can run on a single charge is shorter than in a vehicle having an internal combustion engine as a driving source. One solution for extending the distance that the vehicle can run is to increase the total capacity of the vehicle such as by mounting thereon a plurality of power storage devices, for example.
Another problem with the electric vehicle is to protect a power storage device against overdischarging and overcharging. For example, when the state of charge of the power storage device is low, output power of the electric motor is preferably limited so as to avoid overdischarging. When the state of charge of the power storage device is high, electric power to be charged into the power storage device is preferably limited so as to avoid overcharging. To suitably limit the output power of the electric motor and the electric power to be charged into the power storage device, the state of charge of the power storage device needs to be monitored.
Japanese Patent Laying-Open No. 2010-22128 discloses a charge/discharge control system for a power storage device in which, when any one of a plurality of power storage devices subjected to charge/discharge control as a whole has a state of charge value exceeding a charge limit value, all of the plurality of power storage devices are forcefully discharged, or when any one of the power storage devices has a state of charge value exceeding a discharge limit value, all of the plurality of power storage devices are forcefully charged.
In the case of controlling charging/discharging as a whole, it is necessary to limit charging power or discharging power not to be excessively large. However, even with the total charging power or discharging power into/from the plurality of power storage devices being limited, charging power or discharging power into/from each of the power storage devices is not necessarily limited to a limit value or below. For example, in a system in which a plurality of power storage devices are connected in parallel, a charging current or a discharging current into/from each of the power storage devices may vary when an internal resistance varies. A charging current or a discharging current into/from a power storage device having a low internal resistance may be larger than a charging current or a discharging current into/from a power storage device having a high internal resistance. As a result, charging power or discharging power into/from the power storage device having a low internal resistance may be larger than charging power or discharging power into/from the power storage device having a high internal resistance. In this case, even with charging power or discharging power into/from the plurality of power storage devices as a whole being limited, charging power or discharging power into/from the power storage device having a low internal resistance is not necessarily limited suitably.
In contrast, when limiting electric power in accordance with the state of each power storage device as in the charge/discharge control system disclosed in Japanese Patent Laying-Open No. 2010-22128, electric power is easily limited. This may result in shortage of electric power consumed for the vehicle to run, that is, the driving force of the vehicle.